Every endeavor has been made for higher sensitization and improvement of graininess of a silver halide photographic material. It is known that a sensitizing dye which is used for spectral sensitization largely affects capacities of a silver halide photographic material. A trace of structural difference of a sensitizing dye largely affects photographic capacities such as sensitivity, fog, storage stability and graininess, and the combined use of two or more sensitizing dyes also greatly influences photographic capacities, but it is difficult to foresee its effect. Many engineers have hitherto synthesized various kinds of sensitizing dyes, have examined combinations of a variety of sensitizing dyes and have endeavored to investigate photographic capacities thereby, but it is not possible to know photographic capacities in advance yet.
From the above reasons, techniques of spectrally sensitizing silver halide grains (in particular, reduction sensitized silver halide grains) high sensitively without causing malefic effects such as generation of fog and deterioration of graininess have been demanded.
On the other hand, a trial of reduction sensitization for higher sensitization of a silver halide photographic material has been studied heretofore. Various compounds are disclosed in many patents as useful reduction sensitizers, for example, tin compounds are disclosed in U.S. Pat. No. 2,487,850, polyamine compounds in U.S. Pat. No. 2,512,925, and thiourea dioxide compounds in British Patent 789,823. Further, in Photographic Science and Engineering, Vol. 23, p. 113 (1979), comparisons of natures of silver nuclei formed according to various reduction sensitizing methods are described and dimethylamineborane, stannous chloride, hydrazine, and methods of high pH ripening and low pAg ripening have been employed. Reduction sensitizing methods are also disclosed in U.S. Pat. Nos. 2,518,698, 3,201,254, 3,411,917, 3,779,777 and 3,930,867. Not only the selection of reduction sensitizers but contrivances of reduction sensitizing methods are disclosed in JP-B-57-33572 and JP-B-58-1410 (the term "JP-B" as used herein means an "examined Japanese patent publication").